The use of polycarbonate resins as molding compositions is well known and these materials have been widely employed in the manufacture of thermoplastic molded articles.
In the normal course of the processing of polycarbonate powders to form suitable molding compositions, such powders are usually processed through an extrusion operation either alone or after being blended with other suitable components as may be necessary to satisfy the end use requirements of the molded article to be manufactured.
One particular problem which has faced the industry is the incorporation in the end product of minor amounts of residual volatile impurities which, if not removed during the preparation of the molding composition, can result in various defects in the end product, such as, for instance, blemishes, visual aberrations, or cloudiness in an otherwise clear manufactured article. In addition, the removal of the undesirable volatile impurities also serves to improve the anti-yellowing characteristics of finished products upon heat aging.
Various approaches have been discussed in the prior art to overcome the problem of the removal of residual amounts of volatile impurities from molding compositions with varying degrees of success.
Schnell et al. U.S. Pat. No. 3,267,075 disclose a process for the removal of such impurities via the injection of inert materials.
Skidmore U.S. Pat. No. 3,799,234 has suggested the countercurrent introduction of steam and/or other inert gaseous materials as a method of removing volatile compounds from a plastic material, particularly polyolefins, pointing out that the use of water is contra-indicated for removing residual impurities from polycarbonates since water is not an inert material with respect to polycarbonates which contain chlorides.
It has now been found that the incorporation of a relatively small amount of a volatile matter entraining agent selected from the group consisting essentially of monochlorobenzene, a polyhydric material and solutions of a polyhydric material in water into the polycarbonate resin powder prior to, or during extrusion, acts as a sweeping agent and affects the removal of undesirable residual volatile impurities from the molding composition.